1. Field of the Invention
The present invention relates to a technology of printing dots on a printing media using a dot print head.
2. Description of the Related Art
Printing apparatuses that print using a dot print head that scans in a main scanning direction and in a sub-scanning direction include inkjet printers of serial scanning type and drum scanning type. An inkjet printer forms characters and images on a printing medium by jetting ink from a plurality of print head nozzles. Each of the nozzles of the print head is provided with a pressure chamber charged with ink and an electromechanical conversion element. When an electrical signal is applied to the electromechanical conversion element, pressure is generated in the pressure chamber, causing droplets of ink to be emitted from the nozzle.
One technology for improving the image quality of an inkjet printer is the technology referred to as the interlaced scheme disclosed by U.S. Pat. No. 4,198,642. FIG. 27 illustrates a conventional interlaced printing scheme Print head 1 has eleven nozzles, denoted as nozzles #1 to #11. The nozzle pitch k is six dots in the sub-scanning direction. Here, the unit referred to as a "dot" signifies the minimum dot pitch P[inch] of dots printed on the printing media in the sub-scanning direction; k dots corresponds to k.times.P inches. In FIG. 27, the position of the print head 1 described as pass 1, pass 2 . . . is the position in the sub-scanning direction during main scanning. Here, a "pass" means one main scan. After each main scan, a fixed, sub-scan feed amount F of eleven dots takes place.
In the conventional interlaced printing scheme, the following two conditions are set to ensure the main scanning line (hereinafter also referred to as "raster" or "raster line") is printed with no voids or overlaps. The first condition is that there is a mutual prime integer relationship between the number N of nozzles used and the nozzle pitch k. (Here, "mutually prime" means that there is no common divisor other than 1.) The second condition is that the amount of sub-scan feed amount F equals the number N of nozzles used.
Inkjet printers are subject to the two demands of higher printing speed and better quality. Printing speed can be increased by increasing the number of nozzles used. However, in accordance with the conventional interlaced printing scheme the feed amount F in the sub-scanning direction is set to be the same as the number N of nozzles used, so increasing the number of nozzles also means increasing the sub-scan feed amount F.
However, the mechanical precision of the sub-scan feed is degraded substantially proportionally to the increase in the amount of sub-scan feed. Thus, increasing the number of nozzles degrades the precision of the sub-scan feed. In particular, when a multiplicity of sub-scan feeds are used between adjacent raster lines, the error incurred becomes cumulative and can cause the pitch between adjacent raster lines to deviate from the correct pitch. For example, in the case of FIG. 27 five sub-scan feeds are implemented between No. 5 raster line and No. 6 raster line. Therefore, the pitch between these two raster lines includes the accumulated error of the sub-scan feeds.
FIG. 28 is a more detailed view of the printed dots according to the scheme of FIG. 27. The cumulative error of the sub-scan feeds has increased the pitch of the No. 5 and No. 6 raster lines, resulting in stripe-shaped portions of image degradation that are readily noticeable. This type of image degradation is referred to as "banding." Because banding degrades images, there has always been a desire to reduce banding as much as possible.
In recent years color printers that jet ink of several colors from the print head have come into widespread use for multicolor printing of images processed by computers. Such printers use bidirectional printing technology whereby dots are printed during forward and reverse passes.
In bidirectional printing, dots are printed on a number of raster lines during the forward pass of the print head, and during the reverse pass dots are printed on other raster lines. In accordance with this bidirectional printing method, printing speed can be increased since, compared to a method in which dots are formed only during forward pass by the print head, the bidirectional method doubles the dot printing efficiency.
However, under fixed conditions bidirectional printing has been found to produce images in which the colors are not even. Examples of this phenomenon are illustrated by FIGS. 29 to 31. FIG. 29 shows dots formed at a fixed spacing in the sub-scanning direction using a head equipped with seven nozzles. The spacing of the nozzles is equivalent to four times the printing pitch of the dots in the sub-scanning direction. On the left-hand side of FIG. 29, the numbers 1 to 7 shown in round or diamond-shaped symbols indicate the position of the nozzles in the sub-scanning direction. Specifically, the round symbols indicate the position of the nozzles during the forward pass, and the diamond-shaped symbols indicate the position of the nozzles during the reverse pass. The "1st," "2nd" and so on noted by the nozzles indicate the ordinal number of the main scanning pass by the print head. After each main scan, the paper is moved in the sub-scanning direction by the amount of seven dots. On the right-hand side of FIG. 29 is shown the arrangement of dots printed by the above-described scanning of the print head. Here, circles represent dots formed during the forward pass, and diamonds are dots formed during the reverse pass. Although a diamond shape is used to indicate a position of a nozzle or dot, the nozzles and dots are actually substantially round in shape, the diamond shape being used just to readily differentiate the positions of nozzles and dots. This also applies hereinbelow with respect to descriptions of other drawings.
It will be considered that, as shown in FIG. 29, regions F1, B1, F2 are printed at a uniform hue respectively. FIG. 30 is an enlarged view of dots of region F1 of FIG. 29, formed during a first main scanning pass. The round hatching portions on the right in FIG. 30 indicate dots. To prevent gaps between adjacent raster lines, the dots are formed with a diameter that is slightly larger than the pitch of the dots printed. Dot diameter also depends on the amount of ink that is jetted per unit area of the printing paper. When dot is formed at every pixel on each raster line as shown in FIG. 30, for example, the inkjet amount per unit area is increased and the dot diameter is also increased.
FIG. 31 shows dots formed during a second main scanning pass. The dots formed during the second scanning pass, that is, dots formed during the reverse pass of the print head, are shown as hatched diamonds. As shown, these dots are formed with a large area of overlap with the dots formed by the first main scanning pass (the circles in FIG. 31).
When a print head is used having colored inks arrayed in a row in the main scanning direction, the order in which ink is jetted at the printing position differs from the forward pass to the reverse pass. Take, for example, the print head shown in FIG. 5. FIG. 5 is a plan view of the head, which has six colored inks arrayed from the left in the main scanning direction, the six colors being black (K), cyan (C), light cyan (LC), magenta (M), light magenta (LM) and yellow (Y). When the head is moved to the right (forward pass), with respect to FIG. 5, for a given pixel the inks are jetted in the order of Y, LM, M, LC, C, K. When the head is moved to the left (reverse pass), the inks are jetted in the reverse order. Thus, the forward ink jetting order differs from the reverse ink jetting order, so the order in which the inks penetrate the paper differs. As a result, even if the same quantities of each ink are jetted, there are subtle changes in the hue obtained.
With reference to FIG. 31, dots formed during the second main scanning pass partially overlap the dots formed during the first scanning pass. Based on the effect described above, there is a subtle difference between the hue of dots formed during the first scanning pass and the hue of dots formed during the second scanning pass. It is known that when printing is executed with this subtle difference in hue, the hue of the dots first formed predominates. Thus, in the case of the region F1 in FIG. 29, the predominant hue is that of the dots formed during the forward pass.
Conversely, in the case of region B1 of FIG. 29, it is the hue of the dots formed during the reverse pass that predominates. From FIG. 29, it can be seen that in the case of region B1, the first dots of the second scanning pass ale those formed using nozzles No. 5 to No. 7. Conversely, therefore, to the case of region F1, it is the hue of the dots formed during the reverse pass that forms the dominant hue. In the case of region F2, as in region F1, the predominant hue is again that of the dots formed during the forward pass.
As described above, when bidirectional printing is implemented the difference between the hues of the dots printed during a forward pass and the hues of the dots printed during a reverse pass gives rise to hues that change in terms of unit regions F1, B1, F2. This is perceived as color non-uniformity that degrades the image quality.
For the above reasons, in dot printing there has been a demand for a technology that efficiently prevents image degradation caused by banding or color non-uniformity.